This invention relates to the propulsion of craft and to a fan for providing thrust and pressure in a fluid medium, be it gaseous or liquid. More particularly, this invention relates to a fan for providing thrust and pressure for air-cushion vehicles and watercraft.
Propeller-based propulsion systems typically use axial propellers, as is well known in the case of boats and aircraft. Such propellers comprise a rotating shaft having radially extending blades that are immersed in the fluid to provide propulsion. The propeller blades are typically shaped so as to optimize thrust. The use of airfoil shapes for the blades is common.
Centrifugal fans on the other hand are usually designed to provide airflow under pressure rather than thrust. They are sometimes known as pressure fans. Centrifugal fans according to the prior art comprise an impeller having blades mounted on the disk of the fan or around the periphery of the impeller in a manner similar to a furnace fan. The blades are typically angled so as to draw air inward along the impeller""s rotating axis and discharge it centrifugally in a radial direction where the rapidly moving air is usually collected by a housing and directed to an outlet to provide a pressurized air flow.
The shape of the blades of a centrifugal fan can influence the force exerted on the air, the proportion of energy imparted in the form of air velocity and other factors. For example, Canadian Patent No. 2217829 discloses a centrifugal ventilator exhaust fan that provides high volumetric airflow and generates reduced noise by the use of flat, backwardly inclined and partially curved blades.
U.S. Pat. No. 4,946,348 discloses a centrifugal fan with an impeller comprising a plurality of rearwardly curved blades and a plurality of airfoil vanes positioned axially displaced with respect to the curved blades and located within an annular passageway. The airfoil vanes act to convert the swirl energy of the fluid into pressure.
U.S. Pat. No. 5,279,379 discloses a combined propulsion and lift system for an air-cushion vehicle that includes an axial fan for thrust and a centrifugal fan for cushion pressure wherein both fans are mounted on a common shaft.
Such prior art systems have certain limitations and disadvantages. Conventional centrifugal fans like those described in Canadian Pat. No. 2217829 and in U.S. Pat. No. 4,946,348 provide pressure but no useful thrust. The combined lift-thrust system described in U.S. Pat. No. 5,279,379 simplifies the previous arrangements of fans and propellers used in air-cushion vehicles by providing a common driving source for both the axial propeller and the centrifugal fan. But no prior art approach provides a fan that effectively delivers both pressure and thrust.
It is therefore an object of the present invention to provide a fan that provides both thrust and pressure in the context of a working fluid such as air or water.
One use of such fans for propulsion and cushion pressure is in air-cushion vehicles. Air-cushion vehicles require air pressure to support part or all of their weight over the cushion area. They also require thrust for forward motion and change of direction.
In larger prior art air-cushion vehicles, versions of centrifugal pressure fans are used to provide the cushion pressure airflow for lift. Centrifugal fans basically rely on a momentum change of the air-flow by transforming the mechanical energy input from the engine to kinetic energy in the air in the form of air velocity, and then convert this to pressure energy in the form of a static pressure head in the enclosed lift plenum of the vehicle. The air, and energy, is then dissipated more or less equally around the vehicle and usually no net thrust is provided in any given direction.
The thrust forces for air-cushion vehicles are usually generated by a purely thrust, axial propeller. The thrust propeller differs markedly from a pressure fan. First the fluid flow is into the plane of the propeller, parallel to the axis of rotation. Second, the thrust is provided by the joint action of suction on the leading surface caused by the airfoil shape of the propeller blades, and by the backwash which gives a thrust component by a momentum change to the fluid column. On the better thrust propellers, the suction action can provide up to ⅔ of the total thrust. In water the suction action is not as dramatic but still can account for up to ⅓ of the total thrust of the propeller.
The air-cushion industry thus far has used either two separate systems for the pressure and the thrust, or uses some compromise between the thrust propeller and pressure fan. The present invention allows the advantages of both types to be incorporated in one fan unit to provide a pressurized fluid column for a plenum and a direct thrust.
It is therefore a further object of this invention to provide a fan arrangement in an air-cushion vehicle that provides pressure to a lift plenum as well as thrust for the vehicle.
Another use of this invention is in what are called water jet-drives. In prior art jet drives, the engine drives an auger or screw-type bladed device to force a pressurized column of water into a venturi which converts the pressure to a velocity head. The momentum change in the water column from high pressure to high velocity is what provides the thrust. There is no opportunity to gain direct thrust from a foil reaction on the front surface of the blades. Consequently the overall efficiency of such water jets is limited. This invention provides pressurized water flow into the venturi and at the same time provides a direct thrust from the suction on foil shaped blades to provide higher overall thrusts than the conventional jet drives.
It is therefore a further object of this invention to provide a jet thrust device suitable for such things as propulsion of boats.
Other objects of the invention will be appreciated by reference to the following disclosure.
The invention provides a fan generally arranged like a centrifugal fan with the impeller consisting of a solid disk and having airfoil-shaped blades extending from the perimeter of the disk more or less parallel to the axis of rotation. The propeller-like blades of the fan according to the invention have the airfoil shapes of thrust propellers, necessary for efficient thrust, and the perimetral multi-blade arrangement of pressure fans that traditionally have allowed good pressure generation.
The fan may be used as a fluid propulsion system in a vehicle. The fan is mounted in a housing of the vehicle such that the fluid flow into the fan area is directed to impinge preferentially on one sector of the rotating fan with a component transverse to the blades. The blades in the preferred sector therefore strike the fluid transversely just as they would in an axial flow propeller with a substantial portion of the overall thrust generated by the fan being the result of suction from the advancing surface of the blade. If the flow were equal in each sector of the disk, the suction thrust components from each blade would cancel each other except for small components in the direction of flow that might be present because the blades are at some angle to the disk. When the majority of fluid flow is through one sector, then a significant net thrust is developed into the direction of the fluid flow. If vibrations occur on the fan due to aerodynamic imbalances because of the unequal sector flows, then one way to control these vibrations would be with a tube embedded in the fan around the periphery of the fan and containing lead shot. Such an arrangement is known to control imbalances and this feature is not part of the invention.
The action of the fan blade in the fluid flow is therefore like the blade action of an axial thrust propeller, but the overall fluid flow is similar to the radial flow of prior art centrifugal pressure fans. The fan is able to form efficient compromises between thrust and pressure demands better than either the traditional axial thrust propellers, the centrifugal pressure fans or any known combination of the two.
There are essentially no velocity differences across the flow in the thrust fan according to the invention, as would happen with an axial flow propeller with lower velocities near the centre of the propeller. The fan of the present invention also does not have hub areas in the centre of the flow and can provide high propeller tip speeds (necessary for pressure generation) to the fluid column but with relatively low angular velocity.
The present fan invention is particularly suited to the thrust-cushion vehicle concept disclosed by G. T. Cocksedge in Canadian Patent No. 781,805.
In one aspect, the invention comprises a combined thrust and pressure fan comprising a solid circular disk having an axis of rotation, and a plurality of radially spaced blades extending from the surface of said disk, each of said blades having a generally airfoil shape.
The chord of each of the blades may define an angle in relation to the direction of rotation of the disk so as to define an angle to the tangent of the disk. Preferably that angle is between 10 and 45 degrees.
The blades may also be tilted out of a proper parallel relationship with the axial direction, at an angle of between 0 and 50 degrees. Preferably the tilt is outward in relation to the center of the disk.
In another aspect, the invention comprises a propulsion system for propelling a vehicle for movement in relation to a fluid. The system includes a fan as described above mounted in the vehicle such that the axis of rotation of the disk is in the longitudinal vertical plane of the vehicle and defines an angle of between 0 and 50 degrees in relation to the vertical axis of the vehicle. The orientation of the fan thereby defines a sector of the fan which is aftward in relation to the other sectors of the fan. Means are provided for directing the flow of said fluid such that the flow of said fluid across the fan is preferentially through the aftward sector of the fan and in the rearward direction in relation to the vehicle. The means may comprise shaping of a cowling or vanes for directing the flow of air.
In another aspect, two stages of counter-rotating fans are provided to eliminate the swirl effect on the fluid exiting the fan, to minimize the gyroscopic forces caused by inertial imbalance and to allow a given pressure at lower tip speed.
In another aspect, the invention comprises a thrust and pressure system for an air cushion vehicle. The system includes at least one plenum chamber and an air outlet from the plenum chamber. A fan as described above is mounted for rotation generally forward of the plenum chamber and such that the axis of rotation of the disk defines an angle of between 0 and 50 degrees in relation to the vertical axis of the vehicle, and wherein the orientation of said disk defines an aft sector of the disk in relation to the vehicle. The aft sector is in air communication with the plenum chamber. Means are provided for directing the flow of air preferentially rearward and through the aft sector of the fan.
In another aspect of the invention, the axis of rotation of the disk defines an angle of between 15 and 45 degrees in relation to the vertical axis of the vehicle, the fan is mounted generally in the bow and the air flow is directed to the aftward portion of the fan by a portion of the bow that generally overlays a forward portion of the fan.
In another aspect, vanes are mounted on a cowling overlaying a portion of the fan in order to direct the flow of air to the aftward sector of the fan.
In another aspect, the invention comprises the adaptation of the above propulsion system for immersed propulsion in water. A solid circular disk having a plurality of airfoil-shaped blades mounted thereon is mounted for rotation in the water craft such that the axis of rotation of said disk defines an angle of between 0 and 50 degrees in relation to the vertical axis of the vehicle, and so as to define an aft sector of the disk in relation to the vehicle. The aft sector of the disk is immersed for rotation through the water, and such that the sectors of the disk other than said aft sector are substantially not immersed in the main water flow.
Preferably the immersed propulsion system includes a rotatable housing for selectively directing water flow toward a selected portion of the disk and the disk may be selectively raised or lowered in relation to the vehicle.
In another aspect the invention comprises a propulsion system for propelling a vehicle for movement in relation to a fluid. A fan is mounted for rotation in the vehicle. The fan comprises a solid circular disk, and a plurality of radially spaced blades extend from the surface of the disk, each blade having a generally airfoil shape. The fan is mounted such that a sector of the fan is relatively more aftward than the other sectors of the fan. Means are provided for directing the flow of the fluid across the fan, such that the principal volume of flow of the fluid is through the aftward sector of the fan and in the rearward direction in relation to the vehicle.
In yet another aspect, the fan is mounted such that the axis of rotation of the disk is substantially in the horizontal plane of the vehicle.
In another aspect, the invention comprises a combined thrust and pressure fan comprising a first and second solid circular disks arranged concentrically in relation to one another. Each disk has plurality of radially spaced blades extending from the surface of the disks, and each blade has a generally airfoil shape.
Other aspects of the invention will be appreciated by reference to the detailed description of the preferred and alternative embodiments and to the claims.